This invention relates to improving the adhesion of aramid, preferably poly(p-phenylene terephthalamide) fiber to polymer matrices in reinforced composite structures. Such improvement is demonstrated by increased interlaminar shear strength.
The commercial importance of fiber reinforced composites has long been recognized. Both thermosetting resins such as unsaturated polyester and epoxy resins and thermoplastic resins such as nylon and polycarbonate are commonly used as matrix material for reinforced composites. Glass, carbon and boron fibers are well-known reinforcement materials. While the adhesion between certain reinforcing fibers and matrix polymer is excellent, other combinations require the use of so-called "coupling agents" in order to approach the maximum strength achievable through reinforcement. This technology is highly specialized, and the suitability of any material to improve the adhesion between any particular fiber and any particular matrix is not predictable. For example, certain silanes have been used to couple glass fiber to epoxy resins but silanes are not effective for coupling carbon fibers with phenolics. Further, some coupling agents are unsuitable because of high cost or questionable carcogenicity. Fiber surface treatments have also been used to enhance adhesion between fiber and matrix.
Relatively recently a new high strength aramid fiber, poly(p-phenylene terephthalamide) fiber has come on the market and reinforcement is one of its major end uses. It is widely used as reinforcement in tires, belts and plastics. Many coupling agents were tried with the object of improving the adhesion of this new fiber to unsaturated polyester matrices with limited success. In particular, it was highly desirable to improve the interlaminar shear strength of unsaturated polyester composites reinforced with poly(p-phenylene terephthalamide) fiber. This is discussed in U.S. Pat. No. 4,328,324 (Col. 4 1. 22-25). The present invention is directed to the solution of this problem.